Where did you buy the jellies? They probably came from our partner http://www.exoticaquaculture.com in which case 78F will be right at the top of their temperature range. This could well be the culprit. If the temperature raised above 78F for any length of time it may have damaged the jellies beyond repair and they will slowly decline in health.

To chill the tank you will need a small, peltier thermoelectric chiller like either of these: